The present invention relates to the determination of the geographical location of a target emitter and, in particular, to the determination of the geographical location of a target emitter by the coherent, time integrated measurement of received signal wavefront phase differences through a synthetic aperture defined by the path of a single mobile receiving platform and the reconstruction of the wavefront of the received signal.
There are many circumstances wherein it is necessary or desirable to determine the geographic location of an emitter of electromagnetic radiation, such as a radar system, a communications facility or device or an emergency beacon or transmitter. Typical applications may include, for example, military signal intelligence (SIGINT) and electronic intelligence (ELINT) operations for locating radar or communications facilities, and air, land and sea rescue operations wherein it is necessary to locate an emergency beacon or transmitter, such as used in aircraft and vessels, or communications devices ranging from conventional or emergency radio devices to cell phones.
Such applications and operations are characterized by common requirements that are, in turn, imposed by general, common characteristics of the target emitters to be located and the situations or circumstances under which the target emitters are to be located. For example, the signal transmitted by a target emitter may be of relatively low power, as in the case of emergency beacons or emergency radios, or may be masked, distorted or effectively reduced by terrain or weather conditions, and such conditions may be intentionally imposed in, for example, military or otherwise hostile situations. In addition, the time available or permissible for locating a target emitter may be limited in both military and civil situations, that is, and for example, in military counter-measures operations or in search and rescue operations, and the resources available for target emitter location may be limited.
As such, it is generally necessary or desirable for a system for locating target emitters to be mobile, that is, to be readily transportable into the general geographical location of a target emitter on an aircraft, vehicle or vessel, both to bring the locator system into range of the target emitter and to allow the locator system to search as large an area as possible in the minimum time. It is also desirable that a locator system be transported and employed in and from a single platform, whether an aircraft, vessel or vehicle, as the use of a single platform reduces the system cost, reduces demand on frequently limited resources and allows a greater area or number of areas to be searched when multiple platforms are available. A single platform system also eliminates the complexity and time delays inherent in deploying and coordinating multiple cooperatively operating platforms.
Related problems are that locator system should be capable of determining the geographic location of a target emitter at the greatest possible range, both to reduce the search time and to reduce risk to the locator system in hostile environments, whether due to weather or terrain factors or otherwise hostile factors. The locator system must also be capable of identifying the geographic location of a target emitter with the greatest possible accuracy as insufficient accuracy in locating a target emitter may render counter-measures ineffective in military situations and may unacceptably delay locating or reaching the target emitter in civil situations, such as search and rescue operations, particularly in difficult terrain or weather conditions. In addition, the locator system should be capable of locating as wide a range of target emitter types as possible, and correspondingly over as wide a range of the electromagnetic spectrum as possible, to allow a given locator system to be employed in as wide a range of applications and situations as possible.
The factors and system elements that determine and limit the characteristics and capabilities of an emitter location system, and in particular a single platform, mobile emitter location system, are numerous and inter-related. Two of the primary elements, however, are the receiving element through which target emitter signals are received and the method by which the received signals are used to identify the geographic location of a target emitter.
For example, current methods for single platform emitter location are based upon determining multiple direction finding (DF) bearings, often referred to as DF xe2x80x9ccutsxe2x80x9d, to the target emitter at points along a path traversed by the locator platform, such as the flight path of an aircraft. Each xe2x80x9ccutxe2x80x9d is a determination the gradient, that is, the directional spatial derivative, of the wavefront of a signal emitted by the target emitter and, in theory, indicates the direction of the emitter relative to the locator platform at the point the xe2x80x9ccutxe2x80x9d is taken. Successive DF cuts are used to determine a Line of Bearing (LOB) xe2x80x9cfanxe2x80x9d of DF cuts and the location of the target emitter is taken as the point of intersection of the DF cuts, that is, of the bearings forming the LOB fan. This method has been found to provide reasonable results within certain limitations, but is subject to significant limitations and problems. For example, signal propagation factors between the emitter and the locator system path at various points, such as variations in propagation conditions, local multipath distortions, multiple propagation paths and reflections, will result in significant errors in the measured gradients of the wavefront and this significant errors in the measured bearings between the locator system and the target emitter. In addition, the accuracy of conventional DF/bearing systems is dependent upon the accuracy with which the associated antenna or other signal receiving element can determine a bearing to an emitter, which in turn is dependent upon the characteristics of the antenna, such as the size of the antenna. For such reasons, it has been found that for reasonable and acceptable accuracy the ratio of the distance between target emitter and the locator system and the length of the path traversed by the locator system between bearings must be on the order of 1:1, thereby severely limiting either the accuracy of the method or the range at which locations can be accurately determined, or both.
The limitations of conventional DF methods and basic problems in determining the location of a signal emitter through conventional DF methods may be more clearly understood and illustrated by briefly considering the principles of operation of conventional direction finding methods. Referring therefore to FIG. 1A, a signal emitted by a Signal Source 2 may be viewed as comprised of a series of curved Wavefronts 4 of wavelength xcex radiating from the Signal Source 2. It is well known and understood that the Gradients 6 of Wavefronts 4, that is, the spatial derivatives of the Wavefronts 4, will be essentially normal to the Wavefronts 4 at each point along each Wavefront 4 and, under relatively ideal transmission conditions, will thereby point to the Signal Source 2. A conventional direction finding (DF) system accordingly attempts to determine two or more Lines Of Bearing 8 to the Signal Source 2 by determining the Gradients 6 of Wavefronts 4 at two or more points along any Wavefront 4, as illustrated by Gradients 6A, 6B and 6C. Gradients 6A, 6B and 6C then determine corresponding Lines of Bearing 8A, 8B and 8C to the Signal Source 2 and the crossing point of Lines of Bearing 8 in turn identifies the location of Signal Source 2. As illustrated in FIG. 1B, however, more normal non-ideal transmission conditions, such as multi-path affects, distort Wavefronts 4 so that while the Gradients 6 at various points along any of Wavefronts 4 are normal to a Wavefront 4 at each point, the Gradients 6, as illustrated by Gradients 6D and 6E, are erratic and inconsistent with respect to the location of Signal Source 2. As such, the corresponding Lines of Bearing 8 determined at each point will be in error, so that the location of Signal Source 8 as determined by the Lines of Bearing 8 will be in corresponding error, and it is possible that the Lines of Bearing 8 may point in directions far removed from the actual location of Signal Source 2.
The inherent errors and limitations of a conventional DF system in determining a line of bearing to a signal source are illustrated more explicitly in FIGS. 2A-2G, 3A-3E and 4A-4C wherein FIG. 2A represents an exemplary two element receiving array having a receiving aperture width of h and for a signal having a wavelength of xcex. The line of bearing to a Signal Source 2 relative to the aperture of the receiving array is represented by the angle xcex8DF relative to a line normal to the midpoint of the axis between the receiving elements. The measured line of bearing to the signal source, xcex8MEAS, is represented by the expression of FIG. 2B as being determined the phase difference between the signals received at the two elements of the array, that is, VA and VB, while the error in determining xcex8MEAS, "sgr"xcex8MEAS, may be expressed as in FIG. 2C wherein SNR is the signal to noise ratio of the received signal at the two elements of the aperture. The relationship between xcex8MEAS and xcex8DF may be expressed as in FIG. 2D, wherein the term sin(xcex8DF) may be expressed as xcex8DF, as represented in FIG. 2E, for situations wherein the signal source is located effectively broadside to the aperture of the receiving array. The error in determining xcex8DF as a function of the error in determining xcex8MEAS may then be expressed as in FIG. 2F, and the substitution of the expression for xcex8MEAS from FIG. 2C into the expression of FIG. 2F to yield the expression of FIG. 2G for the error, "sgr"xcex8DF, in determining the line of bearing to the signal source relative to the aperture of the receiving array. As may be seen from the expression of FIG. 2G, therefore, the error, "sgr"xcex8DF, in determining the line of bearing to a signal source in a conventional DF system is an inverse linear function of the width of the aperture and of the square root of the signal to noise ratio.
FIG. 3A, in turn, illustrates the geometry of range determination by conventional DF methods for the example discussed above wherein R represents the range between the signal source and a first point at which a line of bearing to the signal source is determined and d represents the distance along a baseline extending between the first line of bearing point and a second point at which a line of bearing to the signal source is determined. It will be noted that in the example illustrated in FIG. 3A it is assumed, for simplicity and clarity of discussion, that the angle between the first line of bearing and the baseline is a right angle while the angle between the second line of bearing point and the baseline is represented by xcex8DF. As such, the range R between the first line of bearing point and the signal source may be represented by the expression of FIG. 3B and the relationship between R and xcex8DF as a function of the length of the DF baseline, d, may be represented by the expression of FIG. 3B. The error "sgr"P in determining a range R between the first line of bearing point and the signal source as a function the error "sgr"xcex8DF in determining a line of bearing angle between the baseline and the signal source may therefore be represented by the expression of FIG. 3D, which, with the appropriate substations, yields the expression for "sgr"P of FIG. 3E.
It may be seen from the expression of FIG. 3E that the error "sgr"P in determining a range to a signal source using conventional DF methods is again dependent upon the accuracy with which the DF system can resolve an angular line of bearing to the signal source, which is in turn again inversely linearly dependent upon the width of the aperture and inversely dependent upon the square root of the signal to noise ratio. It may also be seen from the expression of FIG. 3E that while the error "sgr"P in determining a range to a signal source is an inverse linear function of the length of the baseline, the error "sgr"P is a direct function of the square of the range between the baseline and the signal source. This dependence of a conventional DF system upon the length of the baseline and the range to the signal source in determining the location of a signal source is illustrated in FIGS. 4A-4C, which illustrate the errors or tolerances in determining the location of a signal source, for range R to baseline length d ratios, R/d, of, respectively, 25 to 1, 5 to 1 and 0.5 to 1.
It will therefore be apparent that the accuracy with which a conventional DF system can determine the location of a signal source is dependent upon the accuracy and resolution with which the system can resolve line of bearing angles to a signal source. As such, significant improvements in the accuracy of determining line of bearing angles in a DF system require a significant increase in the size of the receiving array aperture, an improvement in the signal to noise ratio, maximizing the length of the baseline or minimizing the ratio between range to the signal source and the length of the baseline, and or combination of any or all of these factors. It will also be understood by those of skill in the relevant arts, however, that the signal to noise ratio is largely determined by the received signal and signal propagation conditions, thereby limiting the degree to which the signal to noise ratio may be improved by improvements in the DF system itself. It will also be understood by those of skill in the relevant arts that there are practical limits to the aperture widths that can be achieved, even using synthetic aperture techniques, and to the signal source range to baseline length ratios that can be achieved.
To illustrate the above discussed practical limitations in implementing a conventional DF system, it is frequently desirable or necessary to implement a DF system into a single, mobile platform as it is often not feasible, for economic and physical reasons, for example, to implement DF networks of the required granularity over all areas of interest. The implementation of a locator system into a single, mobile platform, however, conflicts with the requirement to determine accurate bearings to a target emitter, and in particular when the bearings are to be determined over a broad frequency range. For example, a physically large antenna is necessary to provide a sufficiently large baseline receiving aperture for adequate bearing accuracy at lower frequencies, while closely spaced receiving elements are necessary to avoid ambiguities at higher frequencies. These requirements, in turn, result in a physically large antenna of multiple elements that is difficult to implement in a single, mobile platform unless the platform is very large. Although various methods are known to reduce the physical size of an antenna for a given frequency or to match an antenna to a wide range of frequencies, such methods typically increase the cost of the antenna system, allow only a relatively limited range of frequencies, decrease the receiving efficiency and bearing accuracy of the antenna, or are difficult to implement for various reasons. For example, synthetic aperture techniques, which use platform motion to synthesize an apparent aperture larger than the actual physical antenna, may be used to increase the effective size of an antenna. This method, however, has generally been limited to high frequency applications, such as radar, where the path traveled by the platform between the apparent elements is sufficiently short to appear relatively linear.
In summary, therefore, the capability of a conventional DF system to determine the location of a signal source by triangulation of multiple lines of bearing measured from a baseline is primarily dependent upon the accuracy with which the system can determine the angles of the lines of bearing relative to the baseline and the signal source. As described, a conventional DF system determines the angle of a line of bearing relative to the baseline by determining a single factor, that is, the gradient, or spatial derivative, of the wavefront of the signal emitted by the signal source at the point of measurement of the angle of arrival of the signal. In this regard, a conventional DF system may be analogized to a stereoscopic rangefinder comprised of pinhole cameras. As is well known and understood, a pinhole camera has an essentially infinite depth of field and performs the single imaging function of xe2x80x9cray tracingxe2x80x9d, or projecting, light in a straight line from an image onto a plane. A pinhole camera thereby cannot and does not xe2x80x9cfocusxe2x80x9d an image, but can determine the direction of arrival of a beam or ray of light. As such, two such pinhole cameras separated along a known baseline can operate as a stereoscopic rangefinder to determine the location of a light source by determining angle of arrival of light from the source at each camera, and determining the location of the light source as the intersection point of the beams or rays of light received by the cameras.
Again, however, and because the pinhole cameras use only a single factor or element of information regarding the light emitted by the source, that is, the direction of arrival of the light at each camera, the accuracy of a pinhole stereoscopic rangefinder is determined and limited by the accuracy with which the cameras can determine the angle of arrival of the light. For these reasons, and while a pinhole camera may have an effectively infinite depth of field and a wide angle of image capture, images obtained by a pinhole camera typically lack resolution. As in the instance of a conventional DF system, the accuracy of pinhole cameras in determining the angle of arrival of light from a source is, in turn, determined by such factors as the receiving aperture size, that is, the size of the pinhole. Also, and again as in the instance of a conventional DF system, the accuracy of a pinhole stereoscopic system in locating a light source may be improved by increasing the baseline width between the pinhole cameras.
In conclusion, therefore, the resolution of images captured by a pinhole camera and the location of a signal source by a conventional DF system are similarly limited for the same reason, which is that both systems rely entirely upon a single factor, that is, the capability of the system to determine or resolve the angle of arrival of a signal, or ray of light, at a given point. In the instance of a conventional DF system, however, the problem is further compounded in that under normal non-ideal transmission conditions, such as the presence of multi-path affects, the wavefronts of the signal are distorted so that the gradients of a wavefront at a given location may be erratic and inconsistent with respect to the location of the signal source.
The present invention provides a solution to these and other related problems of the prior art.
The present invention is directed to a method and system for the determination of the geographical location of a signal emitter by the coherent, time integrated measurement of received signal wavefront phase differences through a synthetic aperture defined by the path of a single mobile receiving platform and the reconstruction of the wavefront of the received signal.
According to the present invention, the geographical location of a signal emitter is determined by coherently measuring a phase gradient of a signal emitted by the signal emitter at a plurality of measurement points across a measurement aperture wherein each phase gradient measurement includes an amplitude and a phase gradient of the received signal at the measurement point. The measured phase gradient is integrated for each measurement point to determine a corresponding vector wherein each vector has a direction from the measurement point to the signal emitter and an amplitude proportional to the received signal at the measurement point. A figure of merit is determined for each possible location of the signal emitter by integrating each vector with respect to a propagation path between the measurement point of the vector and the possible location of the signal emitter, and the location of the signal emitter is determined as the possible location of the signal emitter having the highest figure of merit.
Further according to the present invention, the measurement aperture is generated by motion of a receiving aperture along the path and the phase gradient measurements are taken continuously and incrementally at a sequence of measurement points across the measurement aperture, so that the incremental and continuous line integration of the measured phase gradient of the signal across the measurement aperture allows the effective reconstruction of a wavefront of the signal emitted by the signal emitter wherein the reconstruction of the wavefront including the curvature of the wavefront. Each vector is determined by integration of the phase gradient measurements over a measurement period as the receiving aperture moves along a segment of the path and has a direction from the corresponding segment of the path to the location of the signal emitter, and the figure of merit for each possible location of the signal emitter is in turn determined for each vector by integration of the vector over an arc length of the path segment over which the phase gradient measurements of the vector were taken and with respect to the propagation path between the path segment and the possible location of the signal emitter.
In the presently preferred embodiment of the present invention, the receiving aperture is generated as a synthetic aperture by motion of at least two receiving elements along a segment of the path. In one implementation of the present invention, the receiving elements are mounted on an airborne platform and on a towed body connected to and towed by the airborne platform and positional with respect to the airborne platform.
An emitter location system embodying the present invention includes a receiving aperture and a receiver connected from the receiving aperture for determination of a phase gradient measurement at each of a sequence of measurement points across a measurement aperture. The measurement aperture is defined by motion of the receiving aperture along a path and each phase gradient measurement includes an amplitude of the received signal and a phase gradient of the received signal across the receiving aperture. A position information source provides position information relating to the geographic location of each measurement point, and a vector processor integrates the phase gradient measurement of each measurement point to determine a corresponding vector for each measurement point wherein each vector has a direction from the corresponding measurement point to the signal emitter and an amplitude proportional to the received signal at the measurement point. A location processor determines a propagation path between each measurement point and each of a plurality of possible geographic locations of the signal emitter and determines a figure of merit for each possible geographic location of the signal emitter wherein each figure of merit is determined by integrating each vector with respect to a propagation path between the measurement point of the vector and the possible location of the signal emitter.